4 Sep
2011
4 Sep
'11
10:15 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 04 September 2011 15:11
To: cctalk at classiccmp.org
Subject: Re: RQDX3 DEVOFFLINE Problems
>
> > It may be the chip, but what are the control inputs doing? Pin 15 is
> > a
> global
> > enable to that chip, if it's high, the chip is disabled, and all
> > ouptus
> are forced
> > high (IIRC). Often this is not used, and pin 15 is tied to ground,
> > but
> it's best to
> > check.
> >
> > Pin 1 selects between the 1 possible inputs for each multiplexer
section.
Is this in the right state to pass the data through?
Just checked the printset, that shows pin 15 tied to ground. Good job
really pin 2 (and pin 3 is ignored), if it's high, pin 4
is th einverse of pin 3
(and pin 2 is
ignored).
as
to
because I have already ordered the part! Thanks
for telling me though,
I will have to be more careful next time.
The expression 'GIGO' applies to logic gates (and other devices) as much complete computers :-). If the input conditons
aren't right, the output
won't
be right either, even if the chip is working
perfectly.
Oops, didn't check that properly! I really should have checked the full
function of the chip.
Pin 1 of the 158 is connected to a signal that is definitely wrong in that
it is Low when it should be High. Tracking this back it is looking like a
failure in a DEC 8837 chip which is listed as a "RECEIVER, BUS, HEX, UN".
This looks like it inverts signals from the main connector. One of the
outputs of this chip is correctly inverted, the other output is not
correctly inverted, and this is the cause of the input to the 158 being
wrong. So it looks like a partial failure of the chip.
Sourcing a replacement for this chip is probably not going to be easy,
unless there is a generic equivalent, anyone know of an equivalent or have a
spare DEC 8837?
Regards
Rob